Single-sensing systems, which utilize electronic activation devices (solid state sensor, microprocessor) to deploy an airbag in the case of a crash, are normally inactive in the "key-off" stage of the vehicle. A "key-off" stage primarily occurs when a vehicle is parked. This means an electronically activated inflatable restraint system could not protect a passenger in a parked car when another vehicle collides into it. The relatively high power consumption of the electronic activation device makes it impractical to keep the system active in the "key-off" stage by continuous connection to the vehicle's battery since a dead battery would be the eventual result of a long-lasting "key-off".
Efforts to develop a single-sensing airbag system that utilizes an electromechanical sensor which would not consume power in the "key-off" stage have not been successful to date due to the inability of such a sensor to possess the crash detection capabilities of an electronic activation device.
If the electromechanical sensor is trimmed in such a way that it can detect all of the crashes which it is supposed to detect according to relevant specifications, then in practice, the sensor is already "over-sensitive". This means that, under certain circumstances, the sensor will trigger the deployment of the airbag when it should not do so.
If the electromechanical sensor is trimmed in such a way that inadvertent deployments are excluded, then the sensor is already so far insensitive that it cannot detect certain real crash situations. Such a sensor is called "under-sensitive".
In a parked vehicle, both types of electromechanical sensors are able to detect all crash situations which are specified because some difficult circumstances, such as pole crashes and rough road conditions, can be excluded.
To solve the problem of providing airbag protection in a single sensing system in the "key-off" stage, a combination of two principles is proposed: 1) the electronic activation device will be active during the "key-on" stage of the vehicle because it possesses the ability to detect borderline crash impulses under very difficult conditions, such as rough road conditions, pole crashes, etc; and 2) an electromechanical sensor will be used in the "key-off" stage since it does not consume any power while on alert and since conditions for detecting a crash are less difficult when the vehicle is stopped, as would typically be the case when in the "key-off" stage. The invention which provides the solution embodying these two principles will be subsequently described in detail.
A preliminary search of the state of the art has revealed a number of US patents, but none is pertinent to the invention, except to the extent that some disclose airbag systems in which several electromechanical crash sensors are connected in a circuit that is continuously connected to the battery so as to be on alert in both the "key-on" and the "key-off" stages and to the extent that some disclose airbag systems in which an electronic activation device is on alert only in the "key-on" stage. For the record, the search developed U.S. Pat. Nos.: 4933570, 4804859, 4740741, 4701628, 4695075, 4384734, 4381829, 4287431, 4275901, and 4222030.
The principles of the invention will be disclosed with reference to two drawing Figures which show two embodiments in schematic form.